The conventional stent functional coating method has proposed the formation of a dual-layer structure and a multi-layer structure through coating, as shown in FIG. 1. However, a stent with a large diameter needs to be compressed into a small-diameter stent and loaded on a thin catheter, called a delivery device, to move to a target site in the body to be treated, and also, in the target site, the stent loaded in the catheter needs to be safely withdrawn from the catheter and then expanded. Therefore, in the preparation step, the stent needs to be smoothly inserted into thin catheter and, also in the procedure step for a patient, the stent needs to be smoothly withdrawn from the catheter, so that the procedure can be successfully completed.
If a smooth withdrawal is not ensured in the procedure step, both an operator and a patient will face a difficult situation.
Therefore, in order to ensure a smooth insertion and withdrawal of the stent, even after the stent is functionally coated and then compressed for the insertion into the thin catheter, the stent should maintain its basic secure volume without an additional stent volume increase causing interference.
For example, a biliary stent with a diameter of 10 mm and a length of 100 mm woven in a hook shape using 0.12-mm diameter Ni—Ti wire, which is a shape-memory alloy, is coated with a single layer of silicon of about 30 μm, and when such a stent is inserted into, and withdrawn (deployed) from, a catheter with an outer diameter of 8 fr (2.64 mm), a force of about 15-20 N is required (further increased due to problems, such as flexion, after the insertion into the body).
However, when a functional coating of only about 10 μm is further applied to the stent, it is almost impossible to compress the stent and load the stent in the thin catheter in the preparation step. Even if the stent is forcibly loaded, the withdrawal (deployment) of the stent is difficult during the procedure, and thus, both the operator and the patient face a difficult situation, such as the delivery device being broken.
The multilayer coating method, wherein coating layers with approximately 5-10 μm per layer are formed in order to give particular functionality, is difficult to apply in an existing stent system, and rather, it is reasonable to develop and apply a novel stent system.
Therefore, in order to apply a particular functional coating to an existing stent system, a technical approach different from the conventional method is needed.
First, a structure of a stent with hooks and crosses formed by weaving a wire material is shown in FIG. 2.
When the stent in a state of FIG. 2 is subjected to a general external coating, a coating form, in which the outer periphery of the wire is very thinly coated and the portions in contact with inner cells are thickly coated, is observed as shown in FIG. 3. For further understanding, a coating form of a section of one wire is shown in FIG. 4.
In this state, if a coating for a dual-layer or multi-layer structure is conducted for an additional functional coating, the coating is implemented on the already formed coating layer as shown in FIG. 5, and thus, the top side of the wire is coated thicker than before and the lateral side of the wire is also coated thickly, making it difficult for the stent to be substantially compressed and loaded in the delivery device.
Even if an existing coating layer is partially coated the repair of a coated portion in the actual stent preparation site, the same problem occurs, causing difficulty in the preparation process in which the stent is loaded in the delivery device.
An ordinary single-layer coating process is controlled such that the coated stent has such a coating thickness as to be compressed and inserted (loaded) in the delivery device.
Since there is a basic coating thickness required to impart specific functionality, the arbitrary reduction of the coating thickness only for the loading convenience in the delivery device may cause secondary clinical problems after the stent has been applied to the patient.
For example, in a single-layer coating applied to a non-vascular stent, a membrane is formed through the coating of silicon or the like in order to mainly prevent the ingrowth of cancer tissues into the stent, and here, the coating thickness is approximately 30 μm.
Here, if the thickness of the coating is reduced to facilitate the insertion of the stent to be loaded in the delivery device (in the catheter), there may be damages, such as tearing of the coated membrane, when the stent is loaded. If the damaged stent is applied to the patient, a coating membrane cannot achieve the intended purpose, resulting in the ingrowth of cancer tissues into the stent, causing a deterioration in mechanical strength of the coating membrane, thus increasing the possibility that the coating membrane is damaged early.
Therefore, in order to proceed with an additional functional coating without degrading the basic performance of the coating layer after the single layer is coated, the in-depth analysis of the structural shape of the stent and the behavior (reactivity) thereof by external force, such as compression, is conducted, and then an additional functional coating area (secure coating area) that will not affect the loading (insertion) and deployment (withdrawal) of the stent is found (ensured), and then a coating process needs to be performed on the basis of the corresponding area.
For this purpose, it is necessary to design a secure coating area by modeling hooks and crosses of the stent and schematizing the state where the hook and the cross areas are compressed and loaded in the delivery device.
That is, the present inventors have recognized the need for a technology capable of ensuring an area for a functional coating without increasing the deployment force required for the loading and withdrawal of the stent, intensively considering a structure of a stent, an insertion tool, and a preparation process, for a functional coating implement technology applicable to an existing stent system other than the multi-layer coating method.
Throughout the entire specification, many papers and patent documents are referenced and their citations are represented. The disclosure of the cited papers and patent documents are entirely incorporated by reference into the present specification, and the level of the technical field within which the present invention falls and the details of the present invention are explained more clearly.